Jonas Botterman

Persistent luminescence in MSi2O2N2:Eu phosphors

In this work we study the persistent luminescence properties of europium-doped alkaline earth silicon oxynitrides (CaSi2O2N2, SrSi2O2N2 and BaSi2O2N2). All compounds show afterglow emission, with an emission spectrum which is similar to the steady state photoluminescence. The afterglow decay time for BaSi2O2N2:Eu and SrSi2O2N2:Eu is about 50 and 100 minutes respectively, while for CaSi2O2N2:Eu the afterglow intensity is very low. Although the persistent luminescence can be induced by ultraviolet light (250-300 nm) in all three phosphors, only for BaSi2O2N2:Eu low energy radiation (350-500 nm) allows filling of the traps responsible for the afterglow.

Persistent phosphor SrAl₂O₄:Eu,Dy in outdoor conditions: saved by the trap distribution.

Persistent phosphors are a specific type of luminescent materials having the unique ability to emit light long after the excitation has ended. They are commonly used as emergency signage in near ideal, isothermal indoor situations. Recently, their energy storage capacity was relied on for outdoor situations, e.g. for glow-in-the-dark road marks and in combination with solar cells and photo catalytic processes. In this work the influence of temperature, illumination intensity and the duration of the night is critically evaluated on the performance of afterglow phosphors. The persistent luminescence of SrAl2O4:Eu,Dy green emitting phosphors is studied under realistic and idealized conditions. It is found that the light output profile is hardly influenced by the ambient temperature in a wide range. This is due to the presence of a broad trap depth distribution, which is beneficial to cover the longer and colder winter nights. Temperature drops during the night are however detrimental. For traffic applications, the total light output of glow-in-the-dark road marks at the end of the night is not sufficient for the studied compound, although re-charging by the cars headlamps partially alleviates this. For energy storage applications, the trap density should be improved and tunneling recombination processes might be needed to overcome overnight temperature drops.

Lanthanide-Assisted Deposition of Strongly Electro-optic PZT Thin Films on Silicon: Toward Integrated Active Nanophotonic Devices

The electro-optical properties of lead zirconate titanate (PZT) thin films depend strongly on the quality and crystallographic orientation of the thin films. We demonstrate a novel method to grow highly textured PZT thin films on silicon using the chemical solution deposition (CSD) process. We report the use of ultrathin (5-15 nm) lanthanide (La, Pr, Nd, Sm) based intermediate layers for obtaining preferentially (100) oriented PZT thin films. X-ray diffraction measurements indicate preferentially oriented intermediate Ln2O2CO3 layers providing an excellent lattice match with the PZT thin films grown on top. The XRD and scanning electron microscopy measurements reveal that the annealed layers are dense, uniform, crack-free and highly oriented (>99.8%) without apparent defects or secondary phases. The EDX and HRTEM characterization confirm that the template layers act as an efficient diffusion barrier and form a sharp interface between the substrate and the PZT. The electrical measurements indicate a dielectric constant of ∼650, low dielectric loss of ∼0.02, coercive field of 70 kV/cm, remnant polarization of 25 μC/cm(2), and large breakdown electric field of 1000 kV/cm. Finally, the effective electro-optic coefficients of the films are estimated with a spectroscopic ellipsometer measurement, considering the electric field induced variations in the phase reflectance ratio. The electro-optic measurements reveal excellent linear effective pockels coefficients of 110 to 240 pm/V, which makes the CSD deposited PZT thin film an ideal candidate for Si-based active integrated nanophotonic devices.

Plasma enhanced atomic layer deposition of Ga2O3 thin films

Amorphous Ga2O3 thin films have been grown on SiO2/Si substrates by atomic layer deposition (ALD) using tris (2,2,6,6-tetramethyl-3,5-heptanedionato) gallium(III) [Ga(TMHD)3] as a gallium source and O2 plasma as reactant. A constant growth rate of 0.1 A per cycle was obtained in a broad temperature range starting from 100 to 400 °C. X-ray photoelectron spectroscopy (XPS) analysis revealed stoichiometric Ga2O3 thin films with no detectable carbon contamination. A double beam – double monochromator spectrophotometer was used to measure the transmittance of Ga2O3 thin films deposited on a quartz substrate and analysis of the adsorption edge yielded a band gap energy of 4.95 eV. The refractive index of the Ga2O3 films was determined from spectroscopic ellipsometry measurements and found to be 1.84 at a wavelength of 632.8 nm. Atomic force microscopic (AFM) analysis showed surface roughness values of 0.15 and 0.51 nm for films deposited at 200 and 400 °C, respectively. Finally, all the films could be crystallized into a monoclinic β-Ga2O3 crystal structure by a post deposition annealing in He as indicated by X-ray diffraction (XRD) measurements.

Evaluating the use of blue phosphors in white LEDs: the case of Sr0.25Ba0.75Si2O2N2:Eu2+

The luminescence properties of the blue emitting phosphor Sr0.25Ba0.75Si2O2N2:Eu2+ are extensively investigated. This oxonitridosilicate phosphor features strong 4f65d1 - 4f7 luminescence originating from the Eu2+ ion, with a narrow emission band peaking at 467 nm and a full width at half maximum of only 41 nm. Thermal quenching of the blue luminescence only sets in above 450 K, making this material an interesting candidate as LED conversion phosphor. The fast decay of the luminescence prevents the phosphor to be susceptible to saturation effects at high excitation fluxes. Furthermore it is proven to be chemically stable against moisture. The only drawback is the relatively low quantum efficiency of the synthesized powder, provisionally preventing this material to be used in applications. In addition, the phosphor features a weak yellow emission band, originating from small domains featuring a different crystal structure. It is shown that the majority of the powder grains only exhibit blue emission. Finally, the spectrum of a white LED, based on a UV pumping LED and three (oxy)nitride phosphors is simulated in order to assess the usefulness of blue phosphors in LEDs for lighting. Only a marginal improvement in terms of color quality can be achieved with a narrow banded phosphor, at the expense of a decrease in luminous efficacy and overall electrical to optical power efficiency.PACS 70 – Condensed Matter: Electronic structure, Electrical, Magnetic, and Optical PropertiesPACS 42.70.-a Optical materials

Persistent luminescence in MSi 2 O 2 N 2 :Eu phosphors

In this work we study the persistent luminescence properties of europium-doped alkaline earth silicon oxynitrides (CaSi2O2N2, SrSi2O2N2 and BaSi2O2N2). All compounds show afterglow emission, with an emission spectrum which is similar to the steady state photoluminescence. The afterglow decay time for BaSi2O2N2:Eu and SrSi2O2N2:Eu is about 50 and 100 minutes respectively, while for CaSi2O2N2:Eu the afterglow intensity is very low. Although the persistent luminescence can be induced by ultraviolet light (250-300 nm) in all three phosphors, only for BaSi2O2N2:Eu low energy radiation (350-500 nm) allows filling of the traps responsible for the afterglow.

Correction: Plasma enhanced atomic layer deposition of Ga2O3 thin films

Correction for ‘Plasma enhanced atomic layer deposition of Ga2O3 thin films’ by Ranjith K. Ramachandran et al., J. Mater. Chem. A, 2014, 2, 19232–19238.

Persistent luminescence in MSi₂O₂N₂:Eu phosphors

In this work we study the persistent luminescence properties of europium-doped alkaline earth silicon oxynitrides (CaSi2O2N2, SrSi2O2N2 and BaSi2O2N2). All compounds show afterglow emission, with an emission spectrum which is similar to the steady state photoluminescence. The afterglow decay time for BaSi2O2N2:Eu and SrSi2O2N2:Eu is about 50 and 100 minutes respectively, while for CaSi2O2N2:Eu the afterglow intensity is very low. Although the persistent luminescence can be induced by ultraviolet light (250-300 nm) in all three phosphors, only for BaSi2O2N2:Eu low energy radiation (350-500 nm) allows filling of the traps responsible for the afterglow.